1. Field of the Invention
The present invention relates to a rear projection type screen for projecting images from a rear face, and more particularly to a rear projection type screen provided with a Fresnel lens and a lenticular lens.
2. Description of the Related Art
Firstly, an outline of a rear projection type television (hereinafter, called xe2x80x9cRPTVxe2x80x9d) using a rear projection type screen is described. FIG. 1 shows a schematic view of a conventional RPTV. As shown in FIG. 1, the liquid-crystal type RPTV comprises, at the least, a liquid crystal projector 1, and a rear projector type screen 2. This rear projection type screen 2 generally comprises a Fresnel lens sheet 21 and a lenticular lens sheet 22. The Fresnel lens sheet 21 has a function of containing the image light from the liquid crystal projector 1 within a prescribed angular range. In this example, a Fresnel lens 21a is provided on the surface of the Fresnel sheet 21 at which the image light is emitted. Furthermore, the lenticular lens sheet 22 has the function of broadening the image light that has passed through the Fresnel lens sheet 21, to a suitable angular range. In this example, a lenticular lens 22a is provided on the surface of the lenticular lens sheet 22 where the image light is incident thereon.
The base material of the Fresnel lens sheet 21 is, for example, made from an acrylic resin, such as polymethyl methacrylate (PMMA), or a copolymer of PMMA with ethyl acrylate, styrene, or methylacrylate, a vinyl chloride resin, polycarbonate, polystyrene, or a high-impact acrylic resin material constituting a rubber component polymerised with or dispersed in acrylic. The lens pattern of the Fresnel lens 22a is, for example, constituted by an ultraviolet rays curing acrylate resin. The lenticular lens sheet 22 is, for example, made from an acrylic resin, such as PMMA, or a copolymer of PMMA with ethyl acrylate or methylacrylate, a vinyl chloride resin, polycarbonate, polystyrene, or a high-impact acrylic resin material constituting a rubber component polymerised with or dispersed in acrylic.
In FIG. 1, the length indicated by PF is the lens pitch of the Fresnel lens 22a, and the length indicated by PL is the lens pitch of the lenticular lens 22a. Furthermore, PS is the pitch of the pixels projected onto the screen 2 by the liquid-crystal projector 1, and here, this equals the pitch of the pixels on the output face of the lenticular sheet 22a. 
FIG. 2 shows an approximate view of a rear projection type screen. This rear projection type screen 2 comprises a Fresnel lens sheet 21 and a lenticular lens sheet 22, as stated previously, the Fresnel lens 22a and lenticular lens 22a being disposed in a mutually facing arrangement. In the diagram, the output face of the lenticular lens sheet 22 is planar, but it is also possible to provide an external light absorbing layer in the non-collecting portion which does not collect light from the lenticular lens 22a. 
FIG. 3 is a schematic diagram of a Fresnel lens sheet 21 used in a rear projection type screen. As shown in this diagram, there exist optical transmission regions and non-transmission regions in the Fresnel lens sheet 21. Furthermore, FIG. 4 is a schematic diagram of a lenticular lens sheet 22 used in a rear projection type screen. As shown in the diagram, there exist optical transmission regions and non-transmission regions. The optical transmission regions are bright and the optical non-transmission regions are dark. It is also known that the optical transmission regions and non-transmission regions of this kind cause characteristic patterns called xe2x80x9cmoirxc3xa9xe2x80x9d patterns to be produced on both lens sheets. Meanwhile, periodic non-transmission regions occur when an image is produced by a light bulb which have many pixels, such as an LCD.
In the prior art, it is known that the moirxc3xa9 pattern generated between a Fresnel lens 22a and a lenticular lens 22a can be reduced by suitable adjustment of the PL/PF ratio relating to the respective lens pitches PL, PF (Japanese Patent Application: Laid-open No. S59-95525, Japanese Patent Application Laid-open No. S60-263932, Japanese Patent Application Laid-open No. H3-149540). Furthermore, in the case of an image produced by a light bulb which have many pixels, such as an LCD, or the like, in order to reduce the moirxc3xa9 pattern generated between the pixels and the lenticular lens 22a, suitable adjustment of the pitch ratio PS/PL, with respect to the pixel pitch and the lenticular lens 22a pitch, is made similarly(Japanese Patent Application Laid-open No. H3-168630, Japanese-Patent Application Laid-open No. S62-236282, Japanese Patent Application Laid-open No. H2-97991). Japanese Patent Application Laid-open No. H2-97991 also discloses adjustment of the pitch ratio between the screen pitch in the cyclical direction, and the pitch of the pixels, but since the cyclical pitch of the screen is identified as the pitch of the lenticular lens, it does not disclose the idea of the present invention. However, if an image projected by a light bulb having pixels is viewed after passing through a lenticular lens and Fresnel lens, then even if the lenticular lens pitch and the Fresnel lens pitch are adjusted, and the lenticular lens pitch and pixel pitch are adjusted, by using the conventional method, there still remains a problem in that a moirxc3xa9 pattern occurs.
In view of the foregoing, it is an object of the present invention to provide a rear projection type screen which reduce the occurrence of moirxc3xa9 patterns.
It is another object of the present invention to provide a method for manufacturing a rear projection type screen which reduce the occurrence of moirxc3xa9 patterns.
According to one aspect of the present invention, for achieving the above-mentioned object, there is provided a rear projection type screen comprising a Fresnel lens and a lenticular lens, for use with a light valve having pixels, wherein the rear projection type screen has such lens pitches that the secondary moirxc3xa9 pitch PMM indicated by the following equation is 3 mm or less, taking the lens pitch of the Fresnel lens as PF (mm), the lens pitch of the lenticular lens as PL (mm), and the pitch of the pixels in the light valve as PS (mm).   PMM  =                    1                              "LeftBracketingBar"                                          1                PM                            -                              1                PS                                      "RightBracketingBar"                    ⁢                      xe2x80x83                              ⁢              xe2x80x83            ⁢      PM        =          1              "LeftBracketingBar"                              i                          PF              ⁢                              xe2x80x83                                              -                      j            PL                          "RightBracketingBar"            
(where i and j are natural numbers)
According to another aspect of the present invention, there is provided a method of manufacturing a rear projection type screen comprising a Fresnel lens and a lenticular lens, for use with a light valve having pixels, comprising: a first step of calculating the moirxc3xa9 pitch of the moirxc3xa9 pattern generated by the Fresnel lens and the lenticular lens, on the basis of the lens pitch of said Fresnel lens and the lens pitch of the lenticular lens; a second step of calculating a further moirxc3xa9 pitch on the basis of the moirxc3xa9 pitch calculated in the first step and the pitch of the pixels in the light valve; a third step of setting the respective lens pitches in such a manner that the value of the moirxc3xa9 pitch calculated at the second step is equal to or less than a prescribed value; and a fourth step of manufacturing the Fresnel lens and the lenticular lens on the basis of the lens pitch values set in the third step.
The above and further objects and novel features of the invention will more fully appear from the following detailed description when the same is read in connection with the accompanying drawings. It is to be expressly understood, however, that the drawings are for purpose of illustration only and are not intended as a definition of the limits of the invention.